Bonding of bodies of refractory hard materials to carbonaceous supports

ABSTRACT

Bodies (3) such as tiles, plates, slabs or bricks of Refractory Hard Material (RHM) or other refractory composites are bonded to the cathodes or to other components, in particular to a carbon cell bottom (1), of a cell for the production of aluminium by electrolysis of a cryolite-based molten electrolyte, made of carbonaceous or other electrically conductive refractory material, by a non-reactive colloidal slurry (4) comprising particulate preformed RHM in a colloidal carrier selected from colloidal alumina, colloidal yttria and colloidal ceria. The slurry usually comprises preformed particulate TiB 2  in colloidal alumina. The bodies (3) are usually TiB 2  --Al 2  O 3  composites. The bonding is achieved simply by applying the slurry and allowing it to dry.

FIELD OF THE INVENTION

The invention relates to methods of bonding bodies of Refractory HardMaterial (RHM) or other refractory composites to the cathodes of cellsfor the production of aluminium by electrolysis of alumina dissolved ina cryolite-based molten electrolyte, which cathodes are made ofcarbonaceous or other electrically conductive refractory materials Theinvention also relates to such cells having bodies of RHM or refractorycomposites bonded to their cathodes, as well as the use of these cellsfor the production of aluminium.

BACKGROUND OF THE INVENTION

Aluminium is produced conventionally by the Hall-Heroult process, by theelectrolysis of alumina dissolved in cryolite-based molten electrolytesat temperatures up to around 950° C. A Hall-Heroult reduction celltypically has a steel shell provided with an insulating lining ofrefractory material, which in turn has a lining of carbon which contactsthe molten constituents Conductor bars connected to the negative pole ofa direct current source are embedded in the carbon cathode substrateforming the cell bottom floor. The cathode substrate is usually ananthracite based carbon lining made of prebaked cathode blocks, joinedwith a ramming mixture of anthracite, coke, and coal tar.

In Hall-Heroult cells, a molten aluminium pool acts as the cathode. Thecarbon lining or cathode material has a useful life of three to eightyears, or even less under adverse conditions. The deterioration of thecathode bottom is due to erosion and penetration of electrolyte andliquid aluminium as well as intercalation of sodium, which causesswelling and deformation of the cathode carbon blocks and ramming mix.In additon, the penetration of sodium species and other ingredients ofcryolite or air leads to the formation of toxic compounds includingcyanides.

Difficulties in operation also arise from the accumulation ofundissolved alumina sludge on the surface of the carbon cathode beneaththe aluminium pool which forms insulating regions on the cell bottom.Penetration of cryolite and aluminium through the carbon body and thedeformation of the cathode carbon blocks also cause displacement of suchcathode blocks. Due to displacement of the cathode blocks, aluminiumreaches the steel cathode conductor bars causing corrosion thereofleading to deterioration of the electrical contact, non uniformity incurrent distribution and an excessive iron content in the aluminiummetal produced.

A major drawback of carbon as cathode material is that it is not wettedby aluminium. This necessitates maintaining a deep pool of aluminium (atleast 100-250 mm thick) in order to ensure a certain protection of thecarbon blocks and an effective contact over the cathode surface. Butelectromagnetic forces create waves in the molten aluminium and, toavoid short-circuiting with the anode, the anode-to-cathode distance(ACD) must be kept at a safe minimum value, usually 40 to 60 mm. Forconventional cells, there is a minimum ACD below which the currentefficiency drops drastically, due to short-circuiting between thealuminium pool and the anode. The electrical resistance of theelectrolyte in the inter-electrode gap causes a voltage drop from 1.8 to2.7 volts, which represents from 40 to 60 percent of the total voltagedrop, and is the largest single component of the voltage drop in a givencell.

To reduce the ACD and associated voltage drop, extensive research hasbeen carried out with Refractory Hard Metals or Refractory HardMaterials (RHM) such as TiB₂ as cathode materials. TiB₂ and other RHM'sare practically insoluble in aluminium, have a low electricalresistance, and are wetted by aluminium. This should allow aluminium tobe electrolytically deposited directly on an RHM cathode surface, andshould avoid the necessity for a deep aluminium pool. Because titaniumdiboride and similar Refractory Hard Metals are wettable by aluminium,resistant to the corrosive environment of an aluminium production cell,and are good electrical conductors, numerous cell designs utilizingRefractory Hard Metal have been proposed, which would present manyadvantages, notably including the saving of energy by reducing the ACD.

The use of titanium diboride and other RHM current-conducting elementsin electrolytic aluminium production cells is described in U.S. Pat.Nos. 2,915,442, 3,028,324, 3,215,615, 3,314,876, 3,330,756, 3,156,639,3,274,093 and 3,400,061. Despite extensive efforts and the potentialadvantages of having surfaces of titanium diboride at the cell cathodebottom, such propositions have not been commercially adopted by thealuminium industry.

The non-acceptance of tiles and other methods of applying layers of TiB₂and other RHM materials on the surface of aluminium production cells isdue to their lack of stability in the operating conditions, in additionto their cost. The failure of these materials is associated withpenetration of the electrolyte when not perfectly wetted by aluminium,and attack by aluminium because of impurities in the RHM structure. InRHM pieces such as tiles, oxygen impurities tend to segregate alonggrain boundaries leading to rapid attack by aluminium metal and/or bycryolite. To combat disintegration, it has been proposed to use highlypure TiB₂ powder to make materials containing less than 50 ppm oxygen.Such fabrication further increases the cost of the already-expensivematerials. No cell utilizing TiB₂ tiles as cathode is known to haveoperated for long periods without loss of adhesion of the tiles, ortheir disintegration. Other reasons for failure of RHM tiles have beenthe lack of mechanical strength and resistance to thermal shock.

Various types of TiB₂ or RHM layers applied to carbon substrates havefailed due to poor adherence and to differences in thermal expansioncoefficients between the titanium diboride material and the carboncathode block.

U.S. Pat. No. 4,093,524 discloses bonding tiles of titanium diboride andother Refractory Hard Metals to a conductive substrate such as graphite.But large differences in thermal expansion coefficients between the RHMtiles and the substrate cause problems.

EP-A 0 164 830 discloses bonding of solid carbide, boride, nitride,silicide and sulfide bodies by laminating a reactant mixture ofprecursors of the materials of the bodies, then heating to initiate anexothermic reaction producing a layer that bonds the bodies together.However, such methods have not been successfully applied in bondingplates or tiles of TiB₂ or like materials to a carbonaceous or otherconductive refractory substrates.

SUMMARY OF THE INVENTION

The invention provides a method of bonding bodies of Refractory HardMaterial (RHM) or other refractory composites to cathodes or othercomponents of cells of different configurations for the production ofaluminium by electrolysis of a molten electrolyte, which cathodes orcomponents are made up of carbonaceous or other electrically conductiverefractory materials, usually carbonaceous material. According to theinvention, the method comprises placing the RHM or refractory compositebodies onto a cell cathode or other component, with a colloidal slurrycomprising particulate preformed RHM in a colloidal carrier selectedfrom colloidal alumina, colloidal yttria and colloidal ceria in betweenthe bodies and the cathode or other component. The slurry is then driedto bond the bodies to the cathode or other component, the dried slurryacting as a conductive thermally-matched glue which provides excellentbonding of the bodies to the cathode or other component.

Based on adherence tests, it is predicted that such bondedaluminium-wettable cathode bodies should provide a service life of from5 to 20 years, depending on the cell operating conditions. This is farlonger than with any prior method of bonding the bodies to the cathode.

Application of the bodies by means of this non-reactive colloidal slurryis very simple. The formation of an adherent interlayer comprising thepre-formed TiB₂ or other refractory composite in the dried colloidensures an adequate bonding while allowing for thermal expansion whenthe cell is brought to operating temperature. The excellent adherence isbelieved to be due to the fact that the bodies of RHM or otherrefractory composites and the relatively "thick" layers of the driedslurry (usually from about 200 to about 1500 micrometer) have verysimilar thermal expansion coefficients.

It should be noted that the use of non-reactive colloidal slurries hasvery suprisingly been found to outperform reactive mixtures which hadpreviously been tried for the same purpose. The reason for this is notknown.

The colloidal slurry usually comprises preformed particulate preformedTiB₂ in colloidal alumina, and the RHM bodies are made of or compriseTiB₂, for instance TiB₂ --Al₂ O₃ composites, in particular the reactionproducts of a mixture of particulate titanium dioxide, boron oxide andaluminium in the molar proportion 3TiO₂ +3B₂ O₃ +10Al mixed with anamount of preformed particulate TiB₂.

The colloidal slurry preferably comprises 5-100 g of TiB₂ per 10 ml ofcolloid. The colloidal slurry may further comprise particulate carbonwhich serves to provide an excellent conductive bond, particularly withcarbonaceous cell bottoms.

In one method of application, the colloidal slurry is applied to thesurface of the cathode and to the faces of the bodies to be bonded, andthe slurry-coated faces of the bodies to be bonded are applied on theslurry-coated face for the cathode.

Alternatively, the colloidal slurry is conveniently applied to the topsurface of a cathode formed by a cell bottom, and the faces of thebodies to be bonded are applied on the slurry-coated top surface of thecell cathode bottom, without having to apply a separate layer of theslurry onto the surfaces of the bodies.

The bodies may be tiles, plates, slabs or bricks of the RHM or otherrefractory composite material, and the slurry may also be appliedbetween adjacent edges of the tiles, plates, slabs or bricks to bondthem together.

The bodies to be bonded may be coated on all faces with the slurry sothat a layer of the dried slurry is deposited also onto the outer activeface of the bodies. Thus, at least one face of the bodies which is notto be bonded to the cathode may be coated with said slurry and/or with areactive slurry comprising precursors of an RHM or other refractorycomposite. When a reactive slurry is applied to such faces of the body,these faces will be coated with an RHM-containing coating formed byreaction.

The cell cathode bottom possibly has recesses for receiving parts of thebodies which are bonded in said recesses by the applied slurry.

After application of the slurry and placing of the tiles, plates, slabsor bricks of the RHM or other refractory composite material, the slurrycan simply be allowed to dry in the ambient air, possibly assisted byblow heating.

The method is particularly advantageous for carbonaceous cell bottomswhich serve as a conductive cell cathode.

The cell cathode bottom may however be coated with a coating containingRHM or other refractory composites onto which the tiles, plates, slabsor bricks of the RHM or RHM composite are placed and bonded by means ofthe colloidal slurry. This is particularly applicable where a conductiverefractory composite material is used as the cell bottom.

Prior to bonding, the RHM or other refractory composite bodies areadvantageously aluminized on the face not to be bonded, for instance byplacing them in contact with molten aluminium preferably in the presenceof a fluxing agent such as a cryolite-alumina flux.

The invention also concerns a cell for the production of aluminium byelectrolysis of a cryolite-based molten electrolyte, comprising a cellbottom cathode made of carbonaceous or other electrically conductiverefractory material to which are bonded bodies of RHM or otherrefractory composites. The cell according to the invention ischaracterized in that the bodies are bonded to the cell bottom cathodeby a dried slurry comprising particulate preformed RHM in a colloidalcarrier selected from colloidal alumina, colloidal yttria and colloidalceria. This cell incorporates the various features set out above indiscussing its method of production. The invention applies toHall-Heroult cells of classic design and other aluminium productioncells of different configurations including those with deep pool anddrained cathode configurations. Thus, bonded bodies ofaluminium-wettable refractory materials may be arranged in a drainedcell configuration, where molten product aluminium is drainedpermanently from the bodies. Alternatively, bonded bodies of refractorymaterial are arranged on a cell bottom cathode in a deep or shallow poolof molten aluminium.

The invention applies to the bonding of tiles, plates, slabs or bricksto cell bottoms and also to cathodes placed in other configurations, aswell as to the side walls and to other components of the cell such asweirs or baffles associated with the cathodic cell bottom.

The invention concerns mainly bodies of TiB₂ or other aluminium-wettablerefractory materials which in use will be in contact with the moltenproduct aluminium and/or with the cryolite-based electrolyte. But theinvention also contemplates bonding the bodies to carbon pieces, withaluminium between such bodies and a current conductor bar, thisaluminium serving to electrically connect the bodies to the conductorbar.

A further aspect of the invention is the use of said cell for theproduction of aluminium by the electrolysis of alumina dissolved inmolten cryolite, where the product aluminium is in contact with saidbodies bonded on the cell bottom cathode.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-section through part of a cell bottomcathode of an aluminium production cell to which a layer of tiles hasbeen bonded in accordance with the invention; and

FIG. 2 is a schematic cross-section through part of another cell bottomcathode of an aluminium production cell having slabs bonded in recessesin the cell bottom.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows part of a carbon cell bottom 1 of a cell for the productionof aluminium by electrolysis of a molten electrolyte. Current issupplied to the cathodic cell bottom 1 by means of one or moretransverse current collector bars 2 made of steel or a suitable alloy.On the top of the cell bottom 1 are bonded tiles 3 of Refractory HardMaterial (RHM) composite material, usually a TiB₂ --Al₂ O₃ compositematerial, made by the method detailed below. The tiles 3 are bonded tothe cell bottom 1 by a dried slurry 4 comprising particulate preformedRHM in finely divided alumina obtained by applying to the upper surfaceof the cell bottom 1 and/or to the undersides of tiles 3, a non-reactivecolloidal slurry comprising particulate preformed TiB₂ in colloidalalumina and allowing the slurry to dry, as explained in greater detailbelow.

The adjacent edges of the tiles 3 are spaced apart by gaps 5 sufficientto accommodate for thermal expansion of the tiles when the cell isbrought to the operating temperature which may be about 950° C. Theaforesaid slurry is also applied in these gaps 5, so as to bond togetherthe edges of the tiles 3 while allowing for thermal expansion.

Usually, the layer of the dried slurry 4 is about 200 to about 1500micrometer thick. The tiles 3 can have any convenient dimensions,usually several millimeters or tens of millimeters thick.

Once the tiles 3 have been bonded onto the cell bottom 1, which involvessimple drying at ambient temperature, possibly assisted by blowing hotair using an air gun, the cell can be filled with aluminium and acryolite-alumina electrolyte and raised to operating temperature by theusual methods.

The bonding of the tiles 3 by the method of the invention resists thethermal stresses during start up. The RHM materials in the tiles 3ensure excellent wetting of the cell bottom by molten aluminum, whichprotects the carbon of cell bottom 1 against attack by electrolytecomponents. Because the tiles 3 remain firmly bonded to the cell bottom1 for extended periods, despite the aggressive environment, the life ofcarbon cell bottoms can be extended from the usual 2-3 years to 5-20years.

FIG. 2 illustrates another possible cell bottom configuration in which acarbon cell bottom 1 has recesses in the form of rectangular grooves 8receiving therein rectangular slabs 6 of RHM composite material, usuallythe TiB₂ --Al₂ O₃ composite material made by the method detailed below,which protrude from the cell bottom 1.

Inside the grooves 8 the bottom parts of the slabs 6 are bonded firmlyby a dried slurry 4 comprising particulate preformed RHM in finelydivided alumina. As before, this dried slurry is obtained by applying tothe insides of grooves 8 and/or to the underneath parts of slabs 6, anon-reactive colloidal slurry comprising particulate preformed TiB₂ incolloidal alumina and allowing the slurry to dry.

The flat upper surface of the carbon cell bottom 1 and the protrudingupper part of the slabs 6 are coated with a layer 7 containing RHMobtained from a colloidal slurry of reactants, as explained above.Alternatively, it is possible to utilize protruding slabs or othershapes of carbonaceous material having the flat upper surface, or anysurface in contact with the cryolite electrolyte, covered with RHMtiles.

When this cell is in operation, the protruding parts of slabs 6 act asdrained cathodes from which the product aluminium flows down onto thealuminium-wettable layer 7 on the cell bottom. Of course, FIG. 2 merelyschematically shows one type of drained cathode configuration. Manyother cell designs and configurations can use the described non-reactiveslurry bonding technique.

The invention will be further described in the followinglaboratory-scale examples.

Plates (and other shapes) of TiB₂ composite materials were prepared bymixing together particulate reactants in the molar ratio 3TiO₂ +3B₂ O₃+10Al together TiO₂ was 99% pure (metals basis; Johnson Matthey, CatalogNumber 11396) with a particle size of 1.5 to 2.0 micrometer. The B₂ O₃was obtained from Messrs Fischer, Catalog Number A76-3. The aluminiumwas -100 mesh or -325 mesh 99.5% pure, from Johnson Matthey. The TiB₂was from Johnson Matthey, Catalog Number 11364.

The powders were mixed and blended for 15 to 30 minutes. Preferably thereaction powders and TiB₂ are mixed in a weight ratio of about 50:50,but this ratio can range from 90:10 to 30:70, usually in the range 40:60to 60:40. The mixed powders are then vibration poured into a die,without segregation during pouring.

The die is pressed at 35 Ksi (=5.43 K/cm²) for 5 minutes. For largeplates, a load release and repressing operation may be used, or the loadapplication may be gently increased over three minutes. Optimal pressingconditions can be determined for each shape and size being manufactured.After ejection from the die, the pressed plate or other shape should nothave any cracks.

The plates are then combusted, for example with a torch in a CO₂atmosphere, or in a furnace under controlled atmosphere. Prior tofiring, very light refractory bricks are placed below and above theplates in order to minimize distorsion during firing.

After firing, the surface is examined for color and for any melting ofthe refractory. Any skin formed by melting should be removed bymachining.

Next, the plates are aluminized, on their face which is to be in contactwith molten aluminium and which is not to be bonded, by contact of thisface with molten aluminium in the presence of a cryolite-alumina flux,as follows. Aluminium chunks are loaded into a crucible and placed in afurnace at 1000° C. until the aluminium has melted. The crucible isremoved from the furnace and the plate inserted into the moltenaluminium. Pre-mixed powders of cryolite and alumina 90/10% by weightare then spread on top of the melt.

The crucible is placed back in the furnace at 1000° C. for 3 to 24hours, as long as is necessary to aluminize the plate surface to therequired degree. Longer times are preferable; shorter times will providea less complete aluminization than for longer times. The required amountof aluminization will depend on whether the plate is to be used ascathode in configurations where it is exposed to cryolite, where fulleraluminization is desirable.

The plate is then removed from the melt. Examination of the surfaceshows that the surface contains aluminium and has slightly increased inthickness. The aluminized surfaces are shiny and well wettable by moltenaluminium.

The plates were then bonded by their non-aluminized face to a carbonblock forming the cathode of a laboratory aluminium production cell asfollows.

A slurry was prepared from a dispersion of 10 g TiB₂, 99.5% pure, -325mesh (<42 micrometer), in 25 ml of colloidal alumina containing about 20weight % of solid alumina. Coatings with a thickness of 150±50 to 500±50micrometer were applied to the faces of the plates and of the carbonblocks to be applied together. Just after the slurry was applied, andwhile still tacky, the slurry-coated faces of the plates were applied onthe slurry-coated blocks and allowed to dry for about 30 minutes.

The above procedure was repeated varying the amount of TiB₂ in theslurry from 5 to 15 g and varying the amount of colloidal alumina from10 ml to 40 ml. Coatings were applied as before. Drying took 10 to 60minutes depending on the dilution of the slurry and the thickness of thecoatings.

In a further series of tests, a sub-layer of the slurries was applied toeach surface and dried or partly dried before applying the next coating.The two parts were applied together while the last coating was stilltacky.

In all cases, after drying the plates adhered strongly to the carbonblocks. The thermal cycle resistance of the bonded plates/blocks wastested by placing them in a furnace at 900° C. for several minutes, thenremoving them, allowing them to cool in air, and reinserting them in thefurnace. This operation was repeated five times. All of the tilesremained adherent to the blocks after this thermal cycling treatment.

Several of the blocks were tested as cathodes in a laboratory aluminiumproduction cell with the bonded plates in a drained-cathodeconfiguration. The cells operated at low cell voltage and the platesremained adherent after long periods of electrolysis without showing anysign of delamination.

In a variation of the invention, the same bonding technique can be usedto bond together pieces of carbonaceous materials.

I claim:
 1. A method of bonding bodies of Refractory Hard Material (RHM)to a cathode or other component of a cell for production of aluminum byelectrolysis of alumina dissolved in a cryolite-based moltenelectrolyte, wherein the cathode or other component is made ofcarbonaceous or other electrically conductive refractory material, themethod comprising the steps of:placing the RHM composite bodies onto thecathode or other component with a colloidal slurry therebetween, saidcolloidal slurry comprising particulate preformed RHM in a colloidalcarrier, said carrier being selected from the group consisting ofcolloidal alumina, colloidal yttria, colloidal ceria and mixturesthereof; and drying the slurry to bond the bodies to the cathode orother component.
 2. The method of claim 1, wherein the colloidal slurrycomprises preformed particulate TiB₂ in colloidal alumina.
 3. The methodof claim 1, wherein the RHM bodies comprise TiB₂.
 4. The method of claim3, wherein the RHM bodies are TiB₂ --Al₂ O₃ composites.
 5. The method ofclaim 4, wherein the RHM bodies are reaction products of a mixture ofparticulate titanium dioxide, boron oxide and aluminium in molarproportion of 3TiO₂ +3B₂ O₃ +10Al mixed with an amount of preformedparticulate TiB₂.
 6. The method of claim 1, wherein the mixturecomprises 5-100 g of TiB₂ per 10 ml of colloid.
 7. The method of claim1, wherein the colloidal slurry further comprises particulate carbon. 8.The method of claim 1, wherein colloidal slurry is applied to a surfaceof the cathode or of the other component and to RHM body surfaces to bebonded, and the slurry-coated RHM body surfaces to be bonded are appliedon the slurry-coated surface of the cathode or other component.
 9. Themethod of claim 1, wherein the colloidal slurry is applied to a topsurface of a cell bottom, and RHM body surfaces to be bonded are appliedon the slurry-coated top surface of the cell bottom.
 10. The method ofclaim 8 or 9, wherein the bodies to be bonded are tiles, plates, slabsor bricks of the RHM, and the slurry is also applied between contactingedges of adjacent tiles, plates, slabs or bricks to bond them together.11. The method of claim 1, wherein the bodies to be bonded are coated onall faces with the slurry.
 12. The method of claim 1, wherein at leastone face of the bodies which is not to be bonded to the cathode or othercomponent is coated with said slurry, with a reactive slurry comprisingprecursors of said RHM, or with a mixture thereof.
 13. The method ofclaim 1, wherein the bodies are bonded to a cell bottom comprisingrecesses receiving parts of the bodies which are bonded in said recessesby the applied slurry.
 14. The method of claim 1, wherein the slurry isallowed to dry in ambient air, assisted by blow heating.
 15. The methodof claim 1, wherein the cathode or other component is carbonaceous. 16.The method of claim 1, wherein the cathode or other component is coatedwith a coating containing the RHM onto which said bodies are placed andbonded.
 17. The method of claim 1, wherein, prior to or after bonding,the RHM bodies are aluminized on a part thereof which is not bonded ornot to be bonded to the cathode or other component.
 18. The method ofclaim 17, wherein the RHM bodies are aluminized by placing them incontact with molten aluminium.
 19. The method of claim 18, wherein thealuminization occurs in the presence of a fluxing agent.